1. Field of the Invention
The present invention relates to a thermoelectric element which performs the conversion between thermal energy and electric energy, and a method of fabricating the thermoelectric element. Further, the present invention relates to a thermoelectric module which is fabricated by employing such thermoelectric elements.
2. Description of the Related Art
A xe2x80x9cthermoelectric phenomenonxe2x80x9d is the general term of the Seebeck effect, the Peltier effect and the Thomson effect, and elements utilizing the phenomenon are called a xe2x80x9cthermoelectric elementxe2x80x9d, a xe2x80x9cthermocouplexe2x80x9d, an xe2x80x9celectronic cooling elementxe2x80x9d, etc. The thermoelectric phenomenon was originally discovered between different kinds of metals, but in recent years, thermoelectric materials of semiconductors have come to be obtained, and conversion efficiencies not observed with metal materials have come to be attained. Elements employing the thermoelectric semiconductor materials are structurally simple, are easy of handling and can maintain stable characteristics, so that their uses in a wide range attract public attention. In particular, since the elements are capable of precise temperature controls at and near the room temperature, researches and developments have been extensively promoted for temperature regulations in optoelectronics, semiconductor lasers, etc., and for applications to local cooling, small-sized refrigerators, etc.
The figure of merit Z indicating the performance of the thermoelectric material is expressed in terms of the specific resistance (resistivity) xcfx81, thermal conductivity xcexa and Seebeck coefficient xcex1 of the material, as follows:
Z=xcex12/xcfx81xcexa
Incidentally, the Seebeck coefficient xcex1 assumes a positive value in a P-type semiconductor material, whereas it assumes a negative value in an N-type semiconductor material. The thermoelectric element should desirably be large in the figure of merit Z.
Meanwhile, in each of the official gazettes of Japanese Patent Applications Laid-open No. 138789/1988, No. 186299/1996 and No. 56210/1998, it is disclosed to enlarge the figure of merit Z in such a way that extrusion molding working which is a kind of plastic deformation working is employed as a method of molding a thermoelectric element (thermoelectric material, thermoelectric transducer, or sintered thermoelectric semiconductor element).
Also, a globular powdery thermoelectric material for use in the fabrication of a thermoelectric element is disclosed in the official gazette of Japanese Patent Application Laid-open No. 293276/1992. Heretofore, a powdery thermoelectric material has been prepared in such a way that an ingot, which is obtained by heat-melting a predetermined raw material and then cooling the resulting melt, is pulverized, and that the resulting powder is classified. With this method, however, a time period expended on the pulverization as well as the classification is long, and the powder is liable to be contaminated. Further, lowering in the available percentage of the thermoelectric material occurs due to the loading of a sieve at the step of the classification. Besides, especially in case of employing a cleavable material, powdery grains after the pulverization are in the shape of scales. Therefore, the flow of the powder worsens to lower a filling rate in the case where a die is filled up with the powder in order to mold it into the thermoelectric material. Simultaneously, since the surface area of each powdery grain is large relative to the volume thereof, the surface of the powder is liable to oxidize. These problems are alleviated by employing the globular powdery material, but the alleviation is not satisfactory yet.
In view of the above, the present invention has for its object to improve a method of manufacturing a globular powdery thermoelectric material and to combine the method with hot extrusion molding or hot upset forging, thereby to provide a method of fabricating a thermoelectric element as enhances the figure of merit, attains homogeneity in plastic deformation and heightens an available percentage, and the thermoelectric element which is fabricated by the fabricating method. A further object of the present invention is to provide a thermoelectric module which is fabricated including such thermoelectric elements.
In order to accomplish the first-mentioned object, a method of fabricating a thermoelectric element as based on the first viewpoint of the present invention, comprises the step of mixing and heat-melting a raw material of predetermined composition; the step of turning the heat-melted material into microglobules by either of scattering and spraying, and then quenching the microglobules, thereby to prepare a globular powdery thermoelectric material; and the step of plastically deforming the thermoelectric material in a hot condition, thereby to bring crystal grains of said thermoelectric material into a crystal orientation affording an excellent figure of merit.
Besides, a method of fabricating a thermoelectric element as based on the second viewpoint of the present invention, comprises the step of mixing and heat-melting a raw material of predetermined composition; the step of turning the heat-melted material into microglobules by either of scattering and spraying, and then quenching the microglobules, thereby to prepare a globular powdery thermoelectric material; the step of uniformizing grain diameters of the globular powdery thermoelectric material; the step of sintering under pressure the globular powdery thermoelectric material of the uniform grain diameters; and the step of plastically deforming the thermoelectric material sintered under pressure, in a hot condition, thereby to bring crystal grains of said thermoelectric material into a crystal orientation affording an excellent figure of merit.
Further, a method of fabricating a thermoelectric element as based on the third viewpoint of the present invention, comprises the step of mixing and heat-melting a raw material of predetermined composition; the step of turning the heat-melted material into microglobules by either of scattering and spraying, and then quenching the microglobules, thereby to prepare a globular powdery thermoelectric material; the step of uniformizing grain diameters of the globular powdery thermoelectric material; the step of cold-compressing the resulting globular powdery thermoelectric material in an atmosphere of either of an inert gas and a reducing gas; and the step of plastically deforming the cold-compressed thermoelectric material in a hot condition, thereby to bring crystal grains of said thermoelectric material into a crystal orientation affording an excellent figure of merit.
In addition, a thermoelectric element according to the present invention is fabricated by employing the above fabricating method. The thermoelectric element may be made from a raw material being a semiconductor of either of P-type and N-type as contains at least two elements selected from the group consisting of Bi, Te, Sb and Se.
Yet in addition, a thermoelectric module according to the present invention comprises a plurality of such P-type thermoelectric elements; a plurality of such N-type thermoelectric elements; first and second substrates which serve to hold the plurality of P-type and N-type thermoelectric elements; a plurality of first electrodes which are formed on the first substrate, and each of which connects the adjacent P-type and N-type thermoelectric elements; and a plurality of second electrodes which are formed on the second substrate, and each of which connects the adjacent P-type and N-type thermoelectric elements.
According to the present invention, a heat-melted material is turned into microglobules by scattering or spraying, and the microglobules are quenched, thereby to prepare a globular powdery thermoelectric material, so that the globular powdery thermoelectric material of high globularity, small grain diameters and small crystal grains can be obtained. Further, hot plastic working is carried out, so that homogeneity in plastic deformation can be attained to heighten a crystal orientation. It is accordingly possible to improve the figure of merit of a thermoelectric element or a thermoelectric module, and to enhance the available percentage of products.